Thermal-pressure hydrolysis and thermal-alkaline hydrolysis of secondary sludge, from the wastewater treatment plant Waßmannsdorf, were compared based on the physical changes of the treated sludges. For this purpose, seven parameters were determined for investigation. These were: Viscosity, particle size distribution, microscopic images, capillary suction time (CST), TR after the laboratory centrifuge test, zeta potential and foaming potential. To measure these parameters, methods were developed and then applied respectively to sludges from both treatments. The thermal-pressure hydrolysis performed better than the thermal-alkaline hydrolysis in each parameter investigation. In particular, the dewaterability of the sludges after digestion, which represents an important cost factor in sewage plant operation, could be improved by thermal-pressure hydrolysis, but not by thermal-alkaline hydrolysis.